Neutral sodium atoms release from the surfaces of the Moon and Mercury induced by meteoroid impacts

In this work, we calculate the neutral Na production rates on the Moon and Mercury, as due to the impacts of meteoroids having an impact probability on the surface that can influence the daily observations of the exosphere: the meteoroids radius range considered for the Moon and Mercury are 10⁻⁸–0.15 and 10⁻⁸–0.10 m, respectively. We also estimate the mass of meteoroids that has impacted the surfaces of the Moon and Mercury in the last 3.8 Gy (after the end of the Late Heavy Bombardment).The results of our model are that (i) the Na production rates are ∼(3–4.9)×10⁴ and ∼(1.8–2.3)×10⁶ atoms cm⁻² s⁻¹, for Moon and Mercury, respectively, and (ii) in the last 3.8 Gy, the mass of meteoroids that has impacted the whole surface of the Moon and Mercury has been 8.86×10¹⁸ and 2.66×10¹⁹ g, respectively.     

Neutral sodium atoms release from the surface of the Moon induced by meteoroid impacts

In this work, we calculate the vapour and neutral Na production rates on the Moon, as due to the impacts of meteoroids in the radius range of 10⁻⁸–0.15 m. We limit our calculations to this size range, since meteoroids with radius larger than 0.15 m have not been found to be important for the production of the exosphere in a time interval comparable with that of the observations.
We have considered a new dynamical model of the flux of meteoroids at the heliocentric distance of the Moon, regarding objects in the radius range of 10⁻²–0.15 m. Instead, the flux of smaller meteoroids (radius range 10⁻⁸–10⁻² m) has been calculated using the two distributions adopted by Cintala and Love & Brownlee.
The results of our model are that (i) the neutral Na production rate is  ∼3–4.9 × 10⁴ atoms cm⁻² s⁻¹  , slightly larger than the previous estimates  (∼2–3 × 10⁴ atoms cm⁻² s⁻¹)  , and (ii) only about 6 per cent of neutral Na is produced by the impacts of meteoroids in the size range 10⁻³–0.15 m, whereas about 94 per cent of the Na comes from the  10⁻⁵–10⁻³ m  size range.     

Solar infrared occultation observations by SPICAM experiment on Mars-Express: Simultaneous measurements of the vertical distributions of H2O, CO2 and aerosol

The infrared AOTF spectrometer is a part of the SPICAM experiment onboard the Mars-Express ESA mission. The instrument has a capability of solar occultations and operates in the spectral range of 1-1.7 μm with a spectral resolution of ∼3.5 cm⁻¹. We report results from 24 orbits obtained during MY28 at Ls 130°-160°, and the latitude range of 40°-55° N. For these orbits the atmospheric density from 1.43 μm CO₂ band, water vapor mixing ratio based on 1.38 μm absorption, and aerosol opacities were retrieved simultaneously. The vertical resolution of measurements is better than 3.5 km. Aerosol vertical extinction profiles were obtained at 10 wavelengths in the altitude range from 10 to 60 km. The interpretation using Mie scattering theory with adopted refraction indices of dust and H₂O ice allows to retrieve particle size (reff∼0.5-1 μm) and number density (∼1 cm⁻³ at 15-30 km) profiles. The haze top is generally below 40 km, except the longitude range of 320°-50° E, where high-altitude clouds at 50-60 km were detected. Optical properties of these clouds are compatible with ice particles (effective radius reff=0.1-0.3 μm, number density N∼10 cm−3) distributed with variance νeff=0.1-0.2 μm. The vertical optical depth of the clouds is below 0.001 at 1 μm. The atmospheric density profiles are retrieved from CO₂ band in the altitude range of 10-90 km, and H₂O mixing ratio is determined at 15-50 km. Unless a supersaturation of the water vapor occurs in the martian atmosphere, the H₂O mixing ratio indicates ∼5 K warmer atmosphere at 25-45 km than predicted by models.     

Monte Carlo modeling of sodium in Mercury’s exosphere during the first two MESSENGER flybys

We present a Monte Carlo model of the distribution of neutral sodium in Mercury’s exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to ∼10⁶-10⁷ cm⁻² s⁻¹, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER flyby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion.     

Total particulate mass in Enceladus plumes and mass of Saturn’s E ring inferred from Cassini ISS images

The eclipse mosaic (PIA08329) of the Saturn system, taken on September 15, 2006 when Cassini was in Saturn’s shadow, contains numerous color images of the Enceladus plume and the E ring at phase angles ranging from 173° to 179°. These forward-scattering observations sample the diffraction peak for particle radii in the 1–5 μm range. The phase angle dependence and total brightness are sensitive indicators of the total mass of solid material in the plume. We fit the data with a variety of particle shapes and size distributions, and find that the median radius of the equivalent-volume sphere is 3.1 μm, with an uncertainty of ±0.5 μm. The total mass of particles in the plume is (1.45 ± 0.5) × 10⁵ kg. We have not considered variations with altitude in the particle size and shape distribution, and we leave that for another paper. We find that the brightness of the E ring varies with position in the orbit, not only because of the viewing geometry, e.g., variations in phase angle, but also because of some unknown intrinsic variability. The total mass of solid material in the E ring is (12 ± 5.5) × 10⁸ kg. For the plume, the production rate of particles – the mass per unit time leaving the vents is 51 ± 18 kg s⁻¹. We estimate that 9% of these particles are escaping from Enceladus, implying lifetimes of ∼8 years for the E ring particles. Based on three comparisons with vapor amounts from ultraviolet spectroscopy, the ice/vapor ratio is in the range 0.35–0.70. This high ratio poses a problem for theories in which particles form by condensation from the gas phase, and could indicate that particles are formed as spray from a liquid reservoir.     

Observations of the sodium tail of Mercury

Cross-sections of the sodium emission tail of Mercury were measured at various distances down the tail when Mercury was moving away from the Sun (true anomaly angles <180°), and again when Mercury was moving towards the Sun (true anomaly angles >180°). As predicted in early modeling studies, significant differences were expected between these two cases, as the result of Doppler shifts to higher solar intensity in the former case, and to lower solar intensity for the latter case. For observations with Mercury moving away from the Sun, the sodium tail was observed out to about 40,000 kilometers (16 Mercury radii, RM) downstream, expanding, on average, at a rate of 1.9±0.3 km/s. The source rates for sodium generation from Mercury into the tail were found to be in the range 2-5×¹⁰²³ atoms/s, corresponding to between 1 and 10% of the estimated total sodium production rate on the planet. The limiting value of radiation acceleration required to produce an observable sodium tail was estimated to be 112±24 cm/s². For observations where Mercury was moving towards the Sun, the emission intensity in the sodium tail decreased very rapidly with distance downstream, disappearing entirely beyond 12,000 (6 RM) kilometers for radiation accelerations of 128.7 and 135.4 cm/s². For smaller radiation accelerations, the sodium tail was not detectable at all, yielding a limiting value for tail generation of about 122±2 cm/s². Interpretation of the limiting radiation acceleration values suggests that the process that generates the sodium tail yields atoms with energies greater than 3 eV. Particle sputtering is the most reasonable source process.     

The first confirmed Perseid lunar impact flash

The first confirmed lunar impact flash due to a non-Leonid meteoroid is reported. The observed Perseid meteoroid impact occurred at 18^h28^m27^s on August 11, 2004 (UT). The selenographic coordinates of the lunar impact flash are 48±1° N and 72±2° E, and the flash had a visual magnitude of ca. 9.5 with duration of about 1/30 s. The mass of the impactor is estimated to have been 12 g based on a nominal model with conversion efficiency from kinetic to optical energy of 2×¹⁰⁻³. Extrapolation of a power law size-frequency distribution fitting the sub-centimeter Perseid meteoric particles to large meteoroids suggests that several flashes should have been observed at this optical efficiency. The detection of only one flash may indicate that the optical efficiency for Perseid lunar impact is much lower, or that the slope of the size distribution differs between large meteoroids and typical sub-centimeter meteoric particles.     

A survey of meteor spectra and orbits: evidence for three populations of Na-free meteoroids

We present a survey of 97 spectra of mainly sporadic meteors in the magnitude range +3 to −1, corresponding to meteoroid sizes 1-10 mm. For the majority of the meteors, heliocentric orbits are known as well. We classified the spectra according to relative intensities of the lines of Mg, Na, and Fe. Theoretical intensities of these lines for a chondritic composition of the meteoroid and a wide range of excitation and ionization conditions were computed. We found that only a minority of the meteoroids show chondritic composition. Three distinct populations of Na-free meteoroids, each comprising ∼10% of sporadic meteoroids in the studied size range, were identified. The first population are meteoroids on asteroidal orbits containing only Fe lines in their spectra and possibly related to iron-nickel meteorites. The second population are meteoroids on orbits with small perihelia (q?0.2 AU), where Na was lost by thermal desorption. The third population of Na-free meteoroids resides on Halley type cometary orbits. This material was possibly formed by irradiation of cometary surfaces by cosmic rays in the Oort cloud. The composition of meteoroids on Halley type orbits is diverse, probably reflecting internal inhomogeneity of comets. On average, cometary dust has lower than chondritic Fe/Mg ratio. Surprisingly, iron meteoroids prevail among millimeter-sized meteoroids on typical Apollo-asteroid orbits. We have also found varying content of Na in the members of the Geminid meteoroid stream, suggesting that Geminid meteoroids were not released from their parent body at the same time.     

Infrared observations of Comet 81P/Wild 2 in 1997

Comet 81P/Wild 2 was observed in the thermal infrared over 6 months during its 1997 perihelion passage. The comet was most active in late February, about 3 months preperihelion; dust production declined by a factor of 3 between February and August. For the GIOTTO Halley dust size distribution, maximum dust production rate was ∼2 × 10⁶ g/s. The comet displayed a 10-μm silicate feature about 25% above the continuum, similar to several other Jupiter-family comets, but much lower than that seen in a number of Oort cloud comets.NASA’s STARDUST sample return mission will encounter P/Wild 2 98 days postperihelion in January 2004. Based on our observations at a similar point in the orbit and the Halley size distribution, we predict that the mass fluence on the spacecraft for a 150 km miss distance will be about 8 × 10⁻⁶ g/cm² for particles up to 1 cm in radius. The corresponding areal coverage will be about 10⁻⁴.     

Observation of Mercury's sodium exosphere during the transit on November 9, 2006

A rare, but normal, astronomical event occurred on November 9th 2006 (JST) as Mercury passed in front of the Sun from the perspective of the Earth. The abundance of the sodium vapor above the planet limb was observed by detecting an excess absorption in the solar sodium line D₁ during this event. The observation was performed with a 10-m spectrograph of Czerny-Turnar system at Domeless Solar Tower Telescope at the Hida Observatory in Japan. The excess absorption was red-shifted by 10 pm relative to the solar line, and was measured at the dawnside (eastside) and duskside (westside) of Mercury. Between the dawn and dusksides, an asymmetry of total sodium abundance was clearly identified. At the dawnside, the total sodium column density was 6.1×10¹⁰ Na atoms/cm², while it was 4.1×10¹⁰ Na atoms/cm² at the duskside. The investigation of dawn-dusk asymmetry of the sodium exosphere of Mercury is a clue to understand the release mechanism of sodium from the surface rock. Our result suggests that a thermal desorption is a main source process for sodium vapor in the vicinity of Mercury.     

A model of interaction of Phobos’ surface with the martian environment

We simulate space weathering of Phobos’ surface due to both sputtering by solar wind ions (H⁺ and He²⁺) and planetary protons, and surface material vaporization by micrometeoroids impact. Assuming an Iron-rich composition of Phobos’ regolith, we find that densities of neutral species (Fe, O, Al, Ca, Mg, Na) in the martian environment are in the range (10⁻⁴-10⁻¹ cm⁻³), and observe an ejecta disk with a radius of 6 martian radii in the equatorial plane and a thickness of 3 martian radii in the perpendicular plane. In order to determine the observability of such species from space based instruments, we also estimate their solar scattering emission line intensities. We conclude that Magnesium would be the only potential candidate suitable for spectral detection.     

Collisional balance of the meteoritic complex

Taking into account meteoroid measurements by in situ experiments, zodiacal light observations, and oblique angle hypervelocity impact studies, it is found that the observed size distributions of lunar microcraters usually do not represent the interplanetary meteoroid flux for particles with masses ?10^?10g. From the steepest observed lunar crater size distribution a “lunar flux” is derived which is up to 2 orders of magnitude higher than the interplanetary flux at the smallest particle masses. New models of the “lunar” and “interplanetary” meteoroid fluxes are presented. The spatial mass density of interplanetary meteoritic material at 1 AU is ～10^?16g/m³. A large fraction of this mass is in particles of 10^?6 to 10^?4 g. A detailed analysis of the effects of mutual collisions (i.e., destruction of meteoroids and production of fragment particles) and of radiation pressure has been performed which yielded a new picture of the balance of the meteoritic complex. It has been found that the collisional lifetime at 1 AU is shortest (～10⁴years) for meteoroids of 10^?4 to 1 g mass. For particles with masses m > 10^?5g, Poynting-Robertson lifetimes are considerably larger than collisional lifetimes. The collisional destruction rate of meteoroids with masses $\text{m ? 10}{}^{\mathrm{?3}}\text{g}$ is about 10 times larger than the rate of collisional production of fragment particles in the same mass range. About 9 tons/sec of these “meteor-sized” (m > 10^?5g) particles are lost inside 1 AU due to collisions and have to be replenished by other sources, e.g., comets. Under steady-state conditions, most of these large particles are “young”; i.e., they have not been fragmented by collisions and their initial orbits are not altered much by radiation pressure drag. Many more micrometeoroids of masses m ? 10^?5g are generated by collisions from more massive particles than are destroyed by collisions. The net collisional production rate of intermediate-sized particles 10^?10g ? m ? 10^?5g is found to be about 16 times larger at 1 AU than the Poynting-Robertson loss rate. The total Poynting-Robertson loss rate inside 1 AU is only about 0.26 tons/sec. The smallest fragment particles (m ? 10^?10g) will be largely injected into hyperbolic trajectories under the influence of radiation pressure (β meteoroids). These particles provide the most effecient loss mechanism from the meteoritic complex. When it is assumed that meteoroids fragment similarly to experimental impact studies with basalt, then it is found that interplanetary meteoroids in the mass range 10^?10g ? m ? 10^?5g cannot be in temporal balance under collisions and Poynting-Robertson drag but their spatial density is presently increasing with time.     

Mechanisms accounting for variations in the proportions of carbonaceous and ordinary chondrites in different mass ranges

The number ratio of carbonaceous to ordinary chondrites (the CC/OC ratio) varies with mass. It is very high (≳90) in small mass ranges (10⁻⁸ to 10⁻¹² kg) among interplanetary dust particles and micrometeorites; it is moderately high (~5 to 30) for 1 to 10 m size fireball meteoroids (with estimated masses between ~10³ and ~10⁶ kg). In the range of most normal-sized meteorite falls (0.01–20 kg), the ratio is low (0.04–0.05); the ratio increases at greater mass ranges: at ≥200 kg, the ratio is 0.09; at ≥500 kg, the ratio is 0.20. The CC/OC ratio also increases from 0.05 to 0.16 for small meteorite finds (10⁻³ to 10⁻⁴ kg). High CC/OC ratios at low and high mass ranges are due to the predominance of CC material in the outer solar system. Small particles from this region spiral into the inner solar system typically in ≤10⁶ years due to Poynting–Robertson drag. Meter-sized meteoroids in this region are affected by Yarkovsky forces, pushing them into resonances where they are efficiently transferred to the inner solar system. Normal-sized meteorites are derived from centimeter-to-decimeter-sized meteoroids that have sluggish drift rates (i.e., they are less affected by the seasonal Yarkovsky effect) compared to larger bodies. Consequently, the centimeter-to-decimeter-sized meteoroids spend more time in interplanetary space (where they are subject to collisions) than larger objects. The greater friability of carbonaceous chondrites relative to ordinary chondrites tends to winnow the carbonaceous chondrites out in this size/mass range during their long interplanetary sojourn, thereby decreasing the CC/OC ratio.     

Atmospheric variations and meteorite production on Mars

Through a combination of aerobraking (drag deceleration) and ablation, meteoroids which enter planetary atmospheres may be slowed sufficiently to soft-land as meteorites. Results of an earlier study suggest that the current 6 mbar atmosphere of Mars is sufficient to aerobrake significant numbers of small (<10 kg) asteroidal-type meteoroids into survivable, low-velocity (<500 m s⁻¹) impacts with the planet's surface. Since rates of meteorite production depend upon the density of Mars's atmosphere, they must also change as the martian climate changes. However, to date, martian meteorite production has received relatively little attention in the literature Here we expand upon our previous work to study martian meteorite production rates and how they depend upon variations of the martian atmosphere, and to estimate the ranges of mass, velocity and entry-angle that produce meteorites. We find that even the current atmosphere of Mars is sufficient to soft-land significant fractions of incident stony and iron objects, and that these fractions increase dramatically for denser martian atmospheres. Therefore, like impact cratering, meteorite populations may preserve evidence of past martian climates.     

New upper limits for hydrogen halides on Saturn derived from Cassini-CIRS data

Far infrared spectra (10-600 cm⁻¹) from Cassini's Composite InfraRed Spectrometer (CIRS) were used to determine improved upper limits of hydrogen halides HF, HCl, HBr, and HI in Saturn's atmosphere. Three observations, comprising a total of 3088 spectra, gave 3σ upper limits on HF, HCl, HBr, and HI volume mole fractions of 8.0×¹⁰⁻¹², 6.7×¹⁰⁻¹¹, 1.3×¹⁰⁻¹⁰, and 1.4×¹⁰⁻⁹, respectively, at the 500 mbar pressure level. These upper limits confirm sub-solar abundances of halide species for HF, HCl, and HBr in Saturn's upper atmosphere—consistent with predictions from thermochemical models and influx of material from meteoroids. Our upper limit for HCl is 16 times lower than the tentative detection at 1.1×¹⁰⁻⁹ reported by Weisstein and Serabyn [Weisstein, E.W., Serabyn, E., 1996. Icarus 123, 23-36]. These observations are not sensitive to the deep halide abundance, which is expected to be enriched relative to the solar composition.     

Dust production from the hypervelocity impact disruption of the Murchison hydrous CM2 meteorite: Implications for the disruption of hydrous asteroids and the production of interplanetary dust

More than half of the C-type asteroids, the dominant type of asteroid in the outer half of the main-belt, show evidence of hydration in their reflectance spectra. In order to understand the collisional evolution of asteroids and the production of interplanetary dust and to model the infrared signature of small particles in the Solar System it is important to characterize the dust production from primary impact disruption events, and compare the disruption of hydrous and anhydrous targets. We performed a hypervelocity impact disruption experiment on an ∼30 g target of the Murchison CM2 hydrated carbonaceous chondrite meteorite, and compared the results with our previous disruption experiments on anhydrous meteorites including Allende, a CV3 carbonaceous chondrite, and nine ordinary chondrites. Murchison is significantly more friable than the ordinary chondrites or Allende. Nonetheless, on a plot of mass of the largest fragment versus specific impact energy, the Murchison disruption plots within the field of the anhydrous meteorites points, suggesting that Murchison is at least as resistant to impact disruption as the anhydrous meteorites, which require about twice the energy for disruption as terrestrial anhydrous basalt targets. We determined the mass-frequency distribution of the debris from the Murchison disruption over a nine order-of-magnitude mass range, from ∼10⁻⁹ g to the mass of the largest fragment produced in the disruption. The cumulative mass-frequency distribution from the Murchison disruption is fit by three power-law segments. For masses >10⁻² g the slope is only slightly steeper than that of the corresponding segment from the disruption of most anhydrous meteorites. Over the range from ∼10⁻⁶ to 10⁻² g the slope is significantly steeper than that for the anhydrous meteorites. For masses <10⁻⁶ g the slopes of both the Murchison and the anhydrous meteorites are almost flat. Thus the Murchison disruption significantly over-produced small fragments (10⁻⁶-10⁻³ g) compared to anhydrous meteorite targets. If the Murchison results are representative of hydrous asteroids, the hydrous asteroids may dominate over anhydrous asteroids in the production of interplanetary dust >100 μm in size, the size of micrometeorites recovered from the polar ices, while both types of asteroids might produce comparable amounts of ∼10 μm interplanetary dust. This would explain the puzzle that polar micrometeorites (>100 μm in size) are similar to hydrous meteorites, while the majority of the ∼10 μm interplanetary dust particles are anhydrous.     

A search for interstellar meteoroids using the Canadian Meteor Orbit Radar (CMOR)

Using the CMOR system, a search was conducted through 2.5 years (more than 1.5 million orbits) of archived data for meteoroids having unbound hyperbolic orbits around the Sun. Making use of the fact that each echo has an individually measured error, we were able to apply a cut-off for heliocentric speeds both more than two, and three standard deviations above the parabolic limit as our main selection criterion. CMOR has a minimum detectable particle radius near 100 μm for interstellar meteoroids. While these sizes are much larger than reported by the radar detections of extrasolar meteoroids by AMOR or Arecibo, the interstellar meteoroid population at these sizes would be of great astrophysical interest as such particles are more likely to remain unperturbed by external forces found in the interstellar medium, and thus, more likely to be traceable to their original source regions. It was found that a lower limit of approximately 0.0008% of the echoes (for the 3σ case) were of possible interstellar origin. For our effective limiting mass of 1×10⁻⁸ kg, this represents a flux of meteoroids arriving at the Earth of 6×10⁻⁶ meteoroids/km²/h. For our 2σ results, the lower limit was 0.003%, with a flux of 2×10⁻⁵ meteoroids/km²/h. The total number of events was too low to be statistically meaningful in determining any temporal or directional variations.     

An experimental study of the formation of an ice crust and migration of water vapor in a comet's upper layers

From recent close encounters with Comets Wild-2 and Tempel 1 we learned that their surfaces are very rugged and no simple uniform layers model can be applied to them. Rather, a glaciological approach should be applied for describing their surface features and behavior. Such intrinsically rugged surface is formed in our large scale experiments, where an agglomerate of ∼200 μm gas-laden amorphous ice particles is accumulated to form a 20 cm diameter and few cm high ice sample. The density, tensile strength and thermal inertia of our ice sample were found to be very close to those found by Deep Impact for Comet Tempel 1: density 250-300 kg m⁻³ vs DI 350-400 kg m⁻³; tensile strength 2-4 kPa vs DI 1-10 kPa; thermal inertia 80 W K⁻¹ m⁻² s^1/2 vs <100 W K⁻¹ m⁻² s^1/2 and <50 W K⁻¹ m⁻² s^1/2. From the close agreement between the thermal inertias measured in our ice sample, which had no dust coverage and that of Comet Tempel 1, we deduce that the low thermal inertia is an intrinsic property of the fluffy structure of the ice as a result of its low density, with an addition by the broken terrain and not due to the formation of a dust layer. Upon warming up of the ice, water vapor migrates both outward into the coma and inward. Reaching cooler layers, the water vapor condenses, forming a denser ice crust, as we show experimentally. We also demonstrate the inward and outward flow of water vapor in the outer ice layers through the exchange between layers of D₂O ice and H₂O ice, to form HDO.     

Models for the Origin of the Quadrantids

We present two scenarios for production of the Quadrantid stream based on two different models for its origin: the extinct model in which 2003EH1 was an active comet that released the dust particles during past 5000 years, stopping its activity abruptly in AD 1488; and the split model; in which a catastrophic disruption of an asteroid at AD 1488 released a large number of dust particles in a single event. We calculate the orbital evolution of test particles released in both cases and derive the resulting size distribution of surviving meteoroids in the current Quadrantid stream in the form of s ^−α ds, where s denotes the radius of a meteoroid. We find α = 3.1 in the extinct model and 2.0 in the split model. In addition, the radius of the surviving meteoroids is derived as s >10 μm in the both models. We propose, based on our estimation of the infrared color ratio for the Quadrantid stream derived from both models, that infrared observations of the Quadrantid stream may determine which origin model is more reasonable.     

Assessment of Kinetic Energy of Meteoroids Detected by Satellite-Based Light Sensors

Radiation energies of bright flashes caused by disintegration of large meteoroids in the atmosphere have been measured using optical sensors on board geostationary satellites. Light curves versus time are available for some of the events. We have worked out several numerical techniques to derive the kinetic energy of the meteoroids that produced the flashes. Spectral opacities of vapor of various types of meteoroids were calculated for a wide range of possible temperatures and densities. Coefficients of conversion of kinetic energy to radiation energy were computed for chondritic and iron meteoroids 10 cm to 10 m in size using radiation–hydrodynamics numerical simulations. Luminous efficiency increases with body size and initial velocity. Some analytical approximations are presented for average conversion coefficients for irons and H-chondrites. A mean value of this coefficient for large meteoroids (1–10 m in size) is about 5–10%. The theory was tested by analyzing the light curves of several events in detail.Kinetic energies of impactors and energy–frequency distribution of 51 bolides, detected during 22 months of systematic observations in 1994–1996, are determined using theoretical values of luminous efficiencies and heat-transfer coefficients. The number of impacts in the energy range from 0.25 to 4 kt TNT is 25 per year and per total surface of the Earth.The energy–frequency distribution is in a rather good agreement with that derived from acoustic observations and the lunar crater record. Acoustic systems have registered one 1 Mt event in 12 years of observation. Optical systems have not detected such an event as yet due to a shorter time of observation. The probability of a 1 Mt impact was estimated by extrapolation of the observational data.